Combine enemy and enemygroup layers

Closes #1089

Changes:
- the old 'enemy' map layer has been removed; the 'enemygroup' layer was
renamed to 'enemy' to replace it
- filenames are no longer used for placing enemies. Instead, use enemy
categories and the level property of groups to place specific enemies
- group level no longer requires a maximum value. The minimum is used
when it's missing
- the group area defaults to 1x1 if not defined
- the number of enemies in a group is 1 by default
- waypoints and wander_area only work for single enemies
- direction specifies the starting direction of the entire group
- requires_(not_)status now applies to enemy groups
Don't allow out-of-bounds values for variables in enemy groups
Replace wander_area with wander_radius

Changes:
- All enemies wander by default with a radius of 4 tiles.
- Since an enemy can either have set waypoints or wander randomly (but
never both), wander-specific code has been removed. Instead, wander
points are added to the waypoint queue. The pause between wandering is
the same as waypoint_pause.
Fix bug where wandering enemies would base pathfinding off player pos
Bypass enemy group level if min & max levels are zero
Fix wandering enemies that can see the player through walls
Merge pull request #1092 from dorkster/map_enemy

Combine enemy and enemygroup layers